Piezoelectric devices, such as piezoelectric quartz resonators, piezoelectric quartz filters and the like, typically include a piece of piezoelectric material adhesively mounted to base or substrate and hermetically sealed with a lid, can, shield or the like. In quartz devices, the piezoelectric quartz element, of necessity, has thin metallic electrodes deposited on the surface of the quartz through which electrical signals are coupled to the piezoelectric quartz material.
Common problems with piezoelectric devices are effectively isolating the piezoelectric element from the effects of mechanical shock and minimizing mechanical stresses across the piezoelectric element due to mounting.
It is often the case where the piezoelectric element and its packaging are made of material having much different coefficients of thermal expansion. This mismatch induces mechanical stresses across the quartz plate over time and with temperature variations. This structure, in turn, can adversely affect the frequency stability and performance of the quartz device. In addition, with the advent of smaller packaging, locating the piezoelectric element within its packaging becomes more important since the quartz plate is physically closer to the package walls and is therefore more susceptible to damage from mechanical shock.
Two main packaging schemes have been used for mounting quartz devices. In one scheme the quartz plate is held in a package along two or more opposing edges. This minimizes possible mechanical shock effects on the crystal, but the mismatch in thermal coefficients between the quartz and the packaging material causes stress across the active resonating area of the quartz plate. One solution to this problem has been to isolate the quartz plate from external mechanical shock by utilization of spring-like standoffs or other compliant mounting schemes in the package. This adds expense to the package and is not readily automatable. Another solution is to adhesively mount the quartz plate directly to a ceramic base. Although this is attainable and automatable, the ceramic base is expensive and detrimental stresses are induced across the quartz plate, which can affect its frequency performance. In addition, wire bonds are sometimes used to eliminate some of the adhesive mounting points, but this requires additional equipment, and added expenses in terms of time and processing.
The other main packaging scheme involves mounting the crystal substantially at one end leaving the rest of the quartz blank free. This so-called cantilever-type "mounting minimizes the stress across the active resonating area of the quartz plate, but leaves the free end of the quartz plate susceptible to breakage or chipping from mechanical shocks. This damage would change the frequency characteristics of the device. One solution to this problem is to manufacture special assembly tooling to precisely center the quartz plate in its package to minimize the possibility of damage when the quartz plate flexes from external mechanical shocks. This tooling is expensive and is not readily automatable. Another solution is to solidly mount one end of the quartz plate directly to a ceramic base. Although this can be done and is manufacturable by automation, the ceramic base is expensive and the free end of the quartz plate actually touches, or is in proximity to, the package making the plate very susceptible to damage from mechanical shock.
A significant portion of the cost of a quartz device is in its packaging. Therefore, scrap costs due to yield losses are to be avoided if at all possible. Previously, the packaging used for surface mount quartz devices was mostly ceramic packaging which has demonstrated good yields and therefore low incurred scrap costs. However, the ceramic structures themselves, though effective, are complicated and have a higher inherent cost. Cost reduction can be achieved if the packaging for surface mount quartz devices can be simplified and automated without sacrificing yield.
There is a need for an improved package that: (i) is low cost; (ii) minimizes the number of processing steps and separate packaging components; (iii) is robust under automation processes; (iv) is robust under mechanical shocks and environmental testing; (v) minimizes problems from differing thermal expansion coefficients between the crystal resonator and the package materials; and (vi) is hermetic. Accordingly, a low cost, automatable, cantilever-type mount for a piezoelectric device which minimizes stress across the piezoelectric element and precisely positions the element substantially equidistant from the package walls would be considered an improvement over the art.